- File
- FileReader and FileWriter
- BufferedReader and BufferedWriter
- PrintWriter
File
The class "File" is a representation of a file or directory. Note that it does not represent the data in a file, it does not allow to read or write into a file.
With a "File" object you can:
- make a new empty file or directory
- list the names of the files contained in a directory
- delete files or directories; note that if a directory is not empty then it can not be deleted
- work with paths
Check if a file exists:
File myFile = new File("/etc/passwd");
System.out.println("File exists: "+myFile.exists()); // Output "File exists: true"
Create a new File:
File myFile = new File("/tmp/testFile.tmp"); // This does not create a new file!
System.out.println("File exists: "+myFile.exists());
boolean isFileCreated = false;
try{
isFileCreated = myFile.createNewFile(); // This creates a new file
}catch(java.io.IOException ex){
System.out.println("IOException: "+ex.getMessage());
}
System.out.println("File created: "+isFileCreated);
System.out.println("File exists: "+myFile.exists());
Output:File exists: false
File created: true
File exists: true
Create a new directory:
File myDir = new File("/tmp/testDirectory"); // This does not create a new directory!
System.out.println("Directory exists: "+myDir.exists());
// We create a new directory
boolean isDirectoryCreated = myDir.mkdir();
System.out.println("Directory created: "+isDirectoryCreated);
System.out.println("Directory exists: "+myDir.exists());
Output:Directory exists: false
Directory created: true
Directory exists: true
Delete a file or a directory:
boolean isFileDeleted = false;
boolean isDirectoryDeleted = false;
// Refer to a file
File myFile = new File("/tmp/testFile.tmp");
// Refer to a directory
File myDir = new File("/tmp/testDirectory");
// We delete a file
if(myFile.exists()){
isFileDeleted = myFile.delete();
}
// We delete a directory
if(myDir.exists()){
isDirectoryDeleted = myDir.delete();
}
System.out.println("File deleted: "+isFileDeleted);
System.out.println("Directory deleted: "+isDirectoryDeleted);
Output:File deleted: true
Directory deleted: true
List files contained in a directory:
File tmpDir = new File("/tmp");
for(String fileName : tmpDir.list()){
System.out.println("- "+fileName);
}
Output:- tmp.FZrqYV5538
- .esd-1000
- a.cc06n7
...
FileReader and FileWriter
"FileReader" and "FileWriter" are low level API classes which allow to read/write data from/to a file. As their API is low level, you will usually use a "FileReader" object by injecting it into a "BufferedReader" object. In same way, you will usually use a "FileWriter" object by injecting it into either a "BufferedWriter" object or a "PrintWriter" object.
Their constructors can accept a "File" object or a String object with the value of file's path.
Eg.
File myFile = new File("/etc/passwd");
try{
FileReader fileReader = new FileReader(myFile);
fileReader.close();
}catch(java.io.IOException ex){
System.out.println("IOException: "+ex.getMessage());
}
Or:
try{
FileReader fileReader = new FileReader("/etc/passwd");
fileReader.close();
}catch(java.io.IOException ex){
System.out.println("IOException: "+ex.getMessage());
}
Write in a file with "FileWriter":
// Lignes to write
String[] lignesToWrite = {"This is line 1", "This is line 2", "This is line 3"};
try{
// If the file does not exist then it will create it
// If the file exists then it clear its contents
FileWriter fileWriter = new FileWriter("/tmp/testFile.tmp");
// We write in the file
for(String ligneToWrite : lignesToWrite){
fileWriter.write(ligneToWrite + System.getProperty("line.separator"));
}
// Flush the contents written in the file and then close the stream
fileWriter.close();
}catch(java.io.IOException ex){
System.out.println("IOException while writting: "+ex.getMessage());
}
Read a file's contents with "FileReader":
try{
// We open the file to read
FileReader fileReader = new FileReader("/tmp/testFile.tmp");
// We read the file character by character until the end of the file.
// Note that "fileReader.read()" returns "-1" if the end of the file has been reached.
int characterToRead = 0;
while( (characterToRead = fileReader.read()) != -1){
System.out.print((char)characterToRead);
}
// We close the stream
fileReader.close();
}catch(java.io.IOException ex){
System.out.println("IOException while reading: "+ex.getMessage());
}
Output:File's contents:
This is line 1
This is line 2
This is line 3
BufferedReader and BufferedWriter
"BufferedReader" and "BufferedWriter" are high level API classes which allow to read/write data from/to a file. They are easier to use and offer better performance than the classes "FileReader" and "FileWriter" especially when the program has to manipulate a large chunk of data (they use an efficient buffer mechanism).
Note that the constructor of "BufferedReader" accept only a subtype of the abstract class "Reader" and the constructor "BufferedWriter" accept only a subtype of the abstract class "Writer".
Write in a file with "BufferedWriter":
// Lignes to write
String[] lignesToWrite = {"This is line 1", "This is line 2", "This is line 3"};
try{
// "BufferedWriter" needs a subtype of the abstract class "Writer"
BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter("/tmp/testFile.tmp"));
// We write in the file
for(String ligneToWrite : lignesToWrite){
bufferedWriter.write(ligneToWrite);
bufferedWriter.newLine();
}
// Flush the contents written in the file and then close the stream
bufferedWriter.close();
}catch(java.io.IOException ex){
System.out.println("IOException while writting: "+ex.getMessage());
}
Read a file's contents with "BufferedReader":
try{
// "BufferedReader" needs a subtype of the abstract class "Reader"
BufferedReader bufferedReader = new BufferedReader(new FileReader("/tmp/testFile.tmp"));
// We read the file line by line until the end of the file
// Note that "bufferedReader.readLine()" returns "null" if the end of the file has been reached.
String ligneToRead = null;
while( (ligneToRead = bufferedReader.readLine()) != null){
System.out.println(ligneToRead);
}
// We close the stream
bufferedReader.close();
}catch(java.io.IOException ex){
System.out.println("IOException while reading: "+ex.getMessage());
}
PrintWriter
"PrintWriter" has been improved significantly in Java 5. It has an easier usage than "BufferWriter" and has better performance. Its constructor is very flexible, it can be instantiated with:
- a String object which contains the value of a file's path
- a "File" object
- a subtype of the abstract class "Writer"
Eg.
// Lignes to write
String[] lignesToWrite = {"This is line 1", "This is line 2", "This is line 3"};
try{
// If the file does not exist then it will create it
// If the file exists then it clear its contents
PrintWriter printWriter = new PrintWriter("/tmp/testFile.tmp");
// We write in the file
for(String ligneToWrite : lignesToWrite){
printWriter.println(ligneToWrite); // Add automatically a new carriage return line ('\n', ...)
}
// Flush the contents written in the file and then close the stream
printWriter.close();
}catch(java.io.IOException ex){
System.out.println("IOException while writting: "+ex.getMessage());
}
